Not applicable.
Not applicable.
The present invention relates generally to sensors, and more particularly to measuring changes in a variable that can be related to capacitance using a switched capacitor technique.
Capacitive sensors may be used to convert variations in many measurable variables, such as acceleration and pressure, to variations in capacitance. Another conversion takes place in that the variations in capacitance are then converted to observable variations in voltage. In some implementations of the prior art, an output voltage is directly proportional to gap changes in the capacitor. Prior art examples of such implementations are described by Y. E. Park and K. D. Wise in a publication entitled xe2x80x9cAn MOS Switched-Capacitor Readout Amplifier for Capacitive Pressure Sensorsxe2x80x9d for the IEEE Custom IC Conference of 1983 at pages 380-384. A second such implementation is described by E. D. Joseph, et al. in a publication entitled xe2x80x9cDesign and Noise Analysis of an Automotive Accelerometerxe2x80x9d for the IEEE ISCAS in 1996. A third such implementation is described by B. E. Boser in a publication entitled xe2x80x9cElectronics for Micromachined Inertial Sensorsxe2x80x9d for the International Conference on Solid-State Sensors and Actuators in 1997 at pages 1169-1172. A shortcoming is evident in these prior art implementations in that it appears that the output voltage is directly proportional to gap changes in the capacitor only when there are small variations in gap distance.
Another example of such a prior art implementation is shown in U.S. Pat. No. 4,656,871 entitled xe2x80x9cCapacitor Sensor and Methodxe2x80x9d issued on Apr. 14, 1987 to Motorola, Inc. (hereinafter xe2x80x9cthe ""871 Patentxe2x80x9d). Switched capacitors are used in this invention to measure changes in a variable. The invention described in the ""871 Patent is directed to a method for converting a measured variable to an electrical output signal. The inventor performs this feat by placing sensor capacitors between the output and inverting input terminal of the operational amplifier, thus placing the sensor capacitor at the feedback loop of the operational amplifier. A difference amplifier is placed at the output stage. However, the invention described in the ""871 Patent results in DC offset voltage that must be corrected. Thus, an additional step of correcting the voltage, i.e., the difference amplifier, is necessary. This circuitry results in a more expensive, complicated product to measure the change in variable.
FIG. 1 is a schematic diagram of a prior art capacitive sensor circuit. Disclosed is a switched capacitor method that places the sensor capacitor at the feedback loop of the operational amplifier. This prior art capacitive sensor is used to convert variations in a measured variable to variations in capacitance. This variation in capacitance is then converted to a variation in an electrical output signal. The prior art implementation results in a constant DC offset voltage that needs to be corrected through a difference amplifier at the output stage.
There is a need for a circuit for measuring changes in capacitor gap using a switched capacitor technique that measures changes in a measurable variable such as pressure for larger variations in capacitor gap distances. There is also a need for a circuit for measuring changes in capacitor gap using a switched capacitor technique that does not result in a constant DC offset voltage that needs to be corrected through a difference amplifier at the output stage.
The present invention solves the needs addressed above. The present invention provides a circuit for measuring changes in a capacitor gap, in terms of voltage, using a switched capacitor technique. The change in the capacitor gap corresponds directly to a change in a measurable variable, such as pressure and acceleration. The change in variable also corresponds to a change in voltage. It is an object of the present invention to provide a circuit for measuring changes in a capacitor gap, and thus voltage, using a switched capacitor technique. The sensor includes circuitry that does not result in a DC offset value, but results in the AC component of the voltage being directly proportional to the change in the variable. A substantially constant voltage is supplied to a node near the sensor capacitance, thereby eliminating the DC offset voltage.
In a first embodiment of the present invention, a sensing circuit measures changes in a measurable variable by correlating these changes to voltage changes. This circuit includes an operational amplifier that can be electrically coupled to various supply voltages via five switches that are controlled by a two-phase nonoverlapping clock. During one phase, a first group of switches close; during a second phase, a second set of switches close. Depending on the phase of the clock, and thus the switches that are closed, three substantially constant DC supply voltages are supplied to various points on the sensing circuit through the connections formed by the switches. The various voltages can be generated with a voltage divider and a unity gain buffer. One supply voltage is applied to a node on the circuit and automatically cancels DC offset voltage. Through a unique relationship between the various supply voltages and the value of a CMOS reference capacitor, the output voltage can be made directly proportional to a change in gap for a sensor capacitor.
In another embodiment of the present invention, the circuit is simplified in that the supply voltages are set to substantially equal values, but some of the supply voltages are negative while the others are positive. This embodiment includes an operational amplifier, two groups of switches closed in different phases by a two-phase nonoverlapping clock, substantially constant supply voltages, a reference capacitor and a sensor capacitor.
In yet another embodiment of the present invention, an operational amplifier is coupled to two capacitors other than the reference capacitor, and one of the capacitors (including a sensor capacitor) is coupled to the inverting input of the operational amplifier at different phases of the clock.
In yet another embodiment of the present invention, a fully differential implementation of the circuit is shown. This embodiment contains additional switches and additional capacitors than other embodiments of the invention. In this embodiment, the capacitance of two reference capacitors are correlated to the change in gap of two sensor capacitors. In this embodiment, a differential output voltage is obtained from the outputs of the operational amplifier. The two sensor capacitors of this embodiment are electrically insulated from each other; but their common plate is moveable in accordance with gap increasing in one of the capacitors and decreasing in the other.
It is an object of the present invention to provide for a circuit that correlates changes in capacitance to changes in a measurable variable for larger variations in capacitor gap distances. Moreover, it is an object of the present invention to provide for a circuit that does not result in a constant DC offset voltage that needs to be corrected. Since there is no DC offset voltage, the dynamic range of the circuit is improved.